Hydraulic fracturing of a rock formation includes the circulation of high pressure liquids into a wellbore, and into fluid contact with a subterranean zone, in order to fracture the rock in the zone. In some cases, increased or enhanced production of hydrocarbons from the subterranean zone can occur due to the fracture of the rock, which creates fluid pathways for the hydrocarbons to flow into the wellbore. A pressure at which a hydraulic fracturing liquid must be circulated to fracture the rock depends, for example, on the mechanical properties of the rock in the subterranean zone.